(1) Field of the Invention
The present invention relates to a developing process using a two-component type developer. More particularly, the present invention relates to a developing process in which an image having an excellent resolving power can be obtained without scattering of the toner or dispersion of the toner on a transfer sheet.
(2) Description of the Related Art
A two-component type developer comprising a magnetic carrier and a toner is widely used in commercial electrophotographic copying machines, and at the development of a charged image, a magnetic brush of this developer is formed on a developing sleeve having magnetic poles arranged in the interior thereof and this magnetic brush is brought into contact with a photosensitive material having the charged image to form a toner image.
In this two-component type developer, the compatibity between the toner and carrier has influences on the requirements that a sufficient density should be obtained, scattering of the toner should be prevented and these characteristics should be maintained for a long time.
In general, as the toner density increases, a high image density can be obtained, but the frictional charge of the toner becomes insufficient and the capacity of coupling with the carrier is reduced, and entrance and exit of the toner in a developing device become violent. Accordingly, a marketed tendency to increase scattering of the toner is observed. Therefore, in the conventional developing process using a two-component type developer, the toner concentration is controlled to a lower level. However, in this case, the developing efficiency is generally low and the density of a solid image area is low.
For an optimum image, it is required that the image density should be high, a fine line image area should be faithfully reproduced and the toner should not be dispersed in the periphery of the image (the dispersion of the toner in the periphery of the image will be referred to as "dispersion of the toner" hereinafter). In general, as the toner concentration increases, a high image density can be obtained, but the amount supplied of the toner in the fine line image area should naturally increase and dispersion of the toner is caused by the unnecessarily supplied toner.
As the means for overcoming this defect, Japanese Unexamined Patent Publication No. 62-63790 proposes a process in which an alternating electric field is formed between the drum and sleeve and the volume occupied by the magnetic carrier is controlled to 1.5 to 30% based on the volume between the drum and sleeve.
In this process, if the charging characteristics of the toner and carrier in the two-component type developer are sufficient, it is expected that even at a high toner concentration, the image density will be high and scattering of the toner will be controlled. However, this is substantially impossible in case of a commercial toner of developing process. More specifically, in the production of a toner, insufficient toner particles not containing a charge-controlling agent or having a low content of the charge-controlling agent should inevitably be formed at a certain ratio in the interior or exterior of a developing device. Furthermore, during the developing operation, toner particles from which the charge-controlling agent has been lost by a mechanical force or in which the content of the charge-controlling agent has been reduced are formed at a certain frequency. Moreover, uncharged toner particles in which a necessary charge is not obtained temporarily because the area ratio of an original is changed or the enviroment is changed are contained in the toner. These insufficient toner particles cause scattering of the toner. Furthermore, as the result of the investigation made by us, it was found that if the ratio of air is increased in the developing zone, an air current flowing from the upper part of the developing zone to the lower part (from the downstream side to the upstream side in the rotation direction) is generated by the mutual rotation of the photosensitive drum and developing sleeve, and scattering of the toner to the exterior of the developing device is caused by this air current.
We further found that in a developing process using a two-component type developer comprising a magnetic carrier and a toner, the flowing state of the developer passing through the developing zone is important, and that if the developing conditions are set within certain ranges relatively to this flowing state, even when the toner concentration is relatively high and a toner containing insufficiently charged particles is used, scattering of the toner can be effectively prevented.